SYMPATHETIC AXONS SURROUND NERVE GROWTH FACTOR-IMMUNOREACTIVE TRIGEMINAL NEURONS - OBSERVATIONS IN MICE OVEREXPRESSING NERVE GROWTH-FACTOR

It has been postulated that the aberrant projection of sympathetic axo ns to individual primary sensory neurons may provide the morphological basis for pain-related behaviors in rat models of chronic pain syndro me, Since nerve growth factor (NGF) can elicit the collateral sproutin g of noradrenergic sympathetic terminals, it might be predicted that N GF plays a role in mediating the sprouting of sympathetic axons into s ensory ganglia. Using a line of transgenic mice overexpressing NGF amo ng glial cells, it was first found that trigeminal ganglia from adult transgenic mice possessed significantly higher levels of NGF protein i n comparison to age-matched wildtype mice; as well, detectable levels of NGF mRNA transgene expression were present in both the ganglia and brain stem. Within the trigeminal ganglia, a small proportion of the s ensory neuronal population stained immunohistochemically for NGF; a hi gher percentage of NGF-positive neurons was evident in transgenic mice , New sympathetic axons extended into the trigeminal ganglia of transg enic mice only and formed perineuronal plexuses surrounding only those neurons immunostained for NGF, In addition, such plexuses were accomp anied by glial processes from nonmyelinating Schwann cells, From these data, we propose that accumulation of glial-derived NGF by adult sens ory neurons and its putative release into the ganglionic environment i nduce the directional growth of sympathetic axons to the source of NGF , namely, the cell bodies of primary sensory neurons. (C) 1998 John Wi ley & Sons, Inc.